The present invention relates to optical sensor protection technology, and more particularly, to protection apparatus that may be interposed between a laser and a sensitive detector that attenuates a high intensity laser beam.
Advanced infrared sensors using sensitive detection elements are highly vulnerable to laser countermeasures. Deployable sensors, such as those used in space applictions, for example, typically require some means of protection from laser countermeasure devices. Incorporation of laser protection devices is an important design issue.
State of the art infrared sensors, such as forward looking infrared systems and range finders, are particularly vulnerable to pulsed laser countermeasures. The optical gain of these systems is typically on the order of 10.sup.6 so that even moderate power laser threats can permanently damage sensitive detector elements upon which scene radiation is focused. Pulsed lasers are difficult to protect against because of their fast rise times and high peak powers. In that respect, a TE (transversely excited) laser is a likely threat because of its low cost and high reliability. Such lasers typically have pulse lengths of 50 nsec to 10 .mu.sec and peak powers of about 1 MW.
Gas plasmas initiated by focused laser radiation are known to attenuate a laser beam along the direction of propagation. This effect is useful in sensor protection in that it may be employed to limit countermeasure laser energy transmitted to sensitive detectors. Prior work has shown the usefulness of particulates to reduce the laser intensity threshold for plasma ignition with subsequent attenuation of the laser radiation. For example, see the article by Lencioni, in Appl. Phys. Lett., 23, 12 (1973). In that article and in other reported work, particulates are mixed with a gas in a test cell and laser radiation is brought to a tight focus within the cell. The plasma discharge takes place at the focus. The problem with this approach is that a large number of particulates must somehow be kept in suspension in the gas to guarantee that the minimum number is suspended within the small focal volume and thus be present to initiate plasma ignition.
Schemes to keep particulates suspended include recirculating fans and flow loops that do not lend themselves to compact geometries, and which use lifetime limiting components. Alternatively, the focal spot size can be made large, but then the laser intensity threshold rises. For example, a threshold of 3.times.10.sup.8 W/cm.sup.2 requires a focal spot size of about 500 .mu.m.sup.2. Therefore, in order to maintain operation of the plasma switch at the relatively low flux level of 10.sup.8 W/cm.sup.2 with a small spot size on the order of 100 .mu.m, a means of injecting particulates directly into the focal volume is needed.
An alternative approach is described in U.S. Pat. No. 5,017,769 issued May 21, 1991 in the names of D. B. Cohn, W. Affleck and G. D. Lawrence, and assigned to the assignee of the present application. That application discloses a protection device that successfully creates a sufficient density of particulates near the focal spot to initiate plasma formation when subjected to a high power laser pulse. This was achieved via electrodes positioned adjacent to the focal point whereby a discharge of particulates from the electrodes are injected into the vicinity of the secondary focal spot. Although the protection device achieves satisfactory performance, it requires external power sources and high voltage circuits to effect particulate production and control.
Accordingly it is an objective of the present invention to provide a simplified gaseous laser power limiter that attenuates applied laser energy. It is an objective to provide a system that is fast reacting, with negligible insertion losses. It is a further objective to provide an automatic system that may be employed in a standby mode, thus requiring no separate threat warning devices. It is yet another objective to provide a system that is passive and thus dispenses with the need for external voltage sources and circuitry.